An electrophotography developing method is a method of making toner particles in a developer closely contact an electrostatic latent image formed on a photoreceptor to develop an image. The developer used therein is classified into a two-component type developer containing toner particles and carrier particles, and a one-component type developer using only toner particles.
A developing method using the two-component type developer containing the toner particles and the carrier particles out of the above developers has conventionally employed a cascade process or the like, but currently employs a magnetic brush process using a magnet roll in most cases.
In the two-component type developer, a carrier particle is a carrying substance which gives a desired charge to the toner particles by being stirred together with the toner particles in a developing box filled with the developer, further transports the toner particles charged in such a manner to the surface of the photoreceptor, and make the toner particles form a toner image on the photoreceptor. The carrier particles left on a developing roll which holds a magnet are returned from the developing roll into the developing box, are mixed/stirred with new toner particles, and are repeatedly used for a fixed period of time.
In contrast to a one-component type developer, the two-component type developer contains the carrier particles which have a function of electrically charging the toner particles and further transporting the toner particles by being mixed/stirred with the toner particles, and has excellent controllability when the developer is designed. Accordingly, the two-component type developer is suitable for a full-color developing apparatus which is required to have high image quality, and a high-speed printing apparatus or the like which is required to reliably keep an image and have durability.
The two-component type developer to be used in such a manner is required to show predetermined values of image characteristics such as an image density, fog, a white spot, a tone and a resolution from an early stage of printing, and to stably keep these characteristics constant during the durable printing period of time. In order to keep these characteristics stable, the two-component type developer needs to make the characteristics of the carrier particles contained therein stable.
As the carrier particles composing the two-component type developer, an iron powder carrier such as an iron powder having the surface covered with an oxide film or an iron powder having the surface coated with a resin has been conventionally used. Such an iron powder carrier is highly magnetized and also has high electroconductivity, and accordingly has an advantage of easily obtaining an image having excellent reproducibility in a solidly shaded area.
However, such an iron powder carrier has a true specific gravity as high as about 7.8, is too highly magnetized, and accordingly tends to produce a fusion bonding of a toner component to the surface of the iron powder carrier, which is a so-called toner spent, by being stirred/mixed with the toner particles in the developing box. When such toner spent occurs, the carrier decreases its effective surface area, and tends to decrease the capability of being charged due to friction with the toner particles.
On the other hand, in the case of a resin-coated iron powder carrier, the surface resin is peeled off by a stress during printing, and a core material (iron powder) is exposed which has high electroconductivity and low breakdown voltage. Thus, the resin-coated iron powder carrier may cause a leak of an electric charge. When such a leak of the electric charge occurs, an electrostatic latent image formed on the photoreceptor is damaged, forms a brush line or the like in a solidly shaded area, and hardly provides a uniform image. For these reasons, an iron powder carrier such as an oxide film iron powder and a resin-coated iron powder is not currently used.
In recent years, a ferrite carrier having a true specific gravity as small as about 5.0 and is also lowly magnetized has been used, as a carrier substituted for an iron powder carrier, or a resin-coated ferrite carrier of which the surface is coated with a resin has been used in many cases. As a result, the life of a developer has been drastically extended.
A general method for producing such a ferrite carrier includes the steps of: mixing predetermined amounts of raw materials for the ferrite carrier; calcinating the mixture, pulverizing the calcined mixture, graining the product and sintering the grains. The calcination step may be omitted according to conditions.
However, the method for producing the ferrite carrier has various problems. Specifically, the above produced ferrite particles are contaminated with deformed ferrite particles originating in cracked particles produced when having crushed a block formed in the sintering step, because the raw material is sintered generally in a form of being charged in a housing through a tunnel kiln in the sintering step which is a step of magnetizing the raw material through a ferritization reaction, and then the shape of the sintered raw material tends to be deformed due to interaction between particles and form the block, though the tendency is particularly more noticeable in ferrite particles with a smaller particle size. Besides, in order to produce the ferrite particles with small sizes and an adequate shape, it is necessary to employ a strengthened pulverization technique. Furthermore, the production method has a problem that the production stability is not sufficient, because the production method needs 12 hours of a sintering period of time including a heating-up period, a holding period at the maximum temperature and a cooling period, and needs to crush the formed block after the sintering step.
In addition, a carrier core material produced by such a sintering method contains not only the cracked particles but also a number of deformed particles, so that it is difficult to form a uniform coating film on such particles even when the particles are coated with a resin. The resin coating film tends to be thick in a recess, and be thin on a salient on the surface of the particle. A part having a thin coating film of the resin tends to expose the carrier core material in a short period of service due to the stress, which cause a leakage phenomenon and a spread of the distribution of a charge amount. Accordingly, it has been difficult to stabilize an image quality in a high grade for a long period of time.
In order to prevent cracking and cutting and reduce deformed particles, it is necessary to prevent the agglomeration of particles when the particles are sintered. It is possible to prevent the agglomeration and reduce the cracked particles and the deformed particles, by sintering the raw materials at a lower temperature because the crushing stress after sintering is lowered.
However, thus produced particle is not preferable in terms of its quality and a production cost, because the particle acquires a porous surface, is slowly charged due to an infiltrating resin, and increases an unnecessary amount of the resin due to the infiltration, which is not economical.
In order to solve such problems, a new method for producing a ferrite carrier has been proposed. For instance, Japanese Patent Laid-Open No. 62-50839 discloses a method for producing a ferrite carrier by passing a blend formed of metallic oxides which are raw materials for forming ferrite, through an atmosphere of high-temperature flames, thereby to instantly ferritize the blend.
However, in this production method the raw material of ferrite may be hardly baked depending on the type of the raw material, because a ratio of oxygen quantity to combustion gas quantity is 3 or less. The production method also is not suitable for producing ferrite with a small particle size of about 20 to 50 μm, which copes with a recent trend of using a carrier having a smaller diameter, and cannot provide a spherical and homogeneous ferrite particle.
In addition, Japanese Patent Laid-Open No. 3-233464 discloses a method for producing a carrier for an electrophotographic developer by melting a raw material of a carrier with a direct-current plasma technique, a high-frequency plasma technique or a hybrid plasma technique.
However, the production method employs expensive gases such as argon and helium, accordingly is extremely economically disadvantageous and is not practical.
As described above, such a method has not been found as to be able to produce a ferrite core material for electrophotography and a resin-coated ferrite carrier which have a high degree of fluidity, have little variation of magnetization intensity, are highly magnetized and are spherical, with excellent cost efficiency and high production stability.